Like many a nerd, I have a certain fascination with locks. This originates in reading the memoir Surely You’re Joking, Mr. Feynman as a teenager, in which Richard Feynman recounts picking locks and cracking combinations while at his day job working on the Manhattan Project at Los Alamos. I currently have a couple of locksport pick sets and training locks that I purchased at hacker conferences over the years (although I’m still terrible at it), and I delight in the YouTube exploits of folks like the LockPickingLawyer.

All of which left me wondering if I could try building an interesting lock. One dodge that lockmakers have long used to challenge would-be plunderers is to conceal the existence of the lock itself. A modern variant of this is the magnetic lock, where a magnet is held or slid against an innocuous portion of a door or box, releasing a catch. The problem with this is that, once you guess the location of the lock, any strong magnet will open it.

So I started thinking about embedding an RFID reader under the surface and resigning myself to keeping track of yet another access card. But then I stumbled across something much more interesting: SparkFun’s US $100 magnetic imaging tile. I realized I could use this to build a magnetic lock that opens with a programmable secret gesture.

1 microcontroller [middle]. An array of Hall-effect sensors detect magnetic fields [bottom].

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